1. Field of the Invention
The present invention relates to a zinc alloy that is substantially free of lead and mercury for use in alkaline batteries and methods for its preparation and use, and more particularly to a zinc alloy that is substantially free of lead and mercury and that contains bismuth and indium and provides improved gassing properties in alkaline batteries and methods for its preparation and use.
2. Description of the Related Art
Alkaline batteries are well known in the art and have been described generally in Battery Reference Book, 2nd Edition, Crompton, T. R., Ed., Chap. 6, pp. 6-1 through 6-5, Butterworth-Heinemann, Jordan Hill, Oxford, UK (1995); and Zhang, X. G., Corrosion and Electrochemistry of Zinc, Chap. 13, pp. 373-391, Plenum Press, New York (1996). The conventional alkaline battery is composed of a cathode, an anode and other components of an electrochemical cell that generate an electric current when the circuit between the cathode and anode is closed. One typical construction is a cup-shaped manganese oxide cathode that forms an outer case for the battery. The inside of the cathode is lined with a paper separator that separates the cathode from a highly alkaline paste or gel anode, but permits electric current exchange with the anode. The anode is commonly made up with zinc powder that has been mixed with a gelling agent in a highly alkaline medium and possibly with other components that are found to be desirable by the battery manufacturer. It is common for the anode to be pierced by a central brass collector pin that serves as the contact for the anode with the external circuit.
Since alkaline batteries of this type are commonly used in computers and other instruments that have electrical contacts and other components that would be harmed by contact with the highly alkaline paste of the battery, and also because such batteries often are handled by persons during installation in flashlights, toys and the like, it is important to prevent any leakage of the alkaline contents of the battery. To prevent leakage, these batteries are commonly sealed.
A common and vexing problem experienced in early sealed alkaline batteries was the generation of gas inside the battery. Such gas generation is believed to be caused in batteries during storage and also during discharge by spontaneous corrosion of the zinc with the resulting generation of hydrogen gas. This gas generation in a sealed battery was found to cause bulging of the case and even to break the seal and allow undesirable leakage of the highly alkaline paste from the battery case.
Early attempts to solve the gassing problem of zinc alkaline batteries revealed that the addition of mercury to the zinc powder substantially reduced the gas generation rate. The presence of lead also seemed to improve gassing properties. As used herein, when it is said that gassing properties of a zinc powder are improved, it is meant that a unit amount of the zinc powder, with the improvement evolves less gas per unit time than the same amount of zinc powder without the improvement when each is exposed to the same environmentxe2x80x94such environment being similar to that found inside an alkaline battery.
More recently, however, both lead and mercury have been found to have undesirable environmental consequences and recent efforts have been directed to find alternative ways to improve the gassing properties of zinc powders in zinc alkaline batteries without the use of either lead or mercury. The use of mercury has been discontinued, for all intents and purposes, and the present objective is to find a way to discontinue the use of lead as well.
It has been found that such metals as cadmium, indium, bismuth and others provided a high hydrogen overpotential and reduced gassing when they were added alone, or in combination, to zinc powders. The addition of indium and/or bismuth and magnesium and/or lithium to a zinc anode was disclosed in U.S. Pat. No. 4,994,333. The addition of the metals was reported to suppress zinc corrosion and to improve the mechanical tensile strength of the anode. In U.S. Pat. No. 5,240,793, the addition of indium and bismuth to a zinc powder was reported to provide an alkaline battery with low hydrogen gas evolution. Sugihara et al., in U.S. Pat. No. 5,384,214, reported that addition of yttrium hydroxide prepared in a certain manner to the alkaline electrolyte that was in contact with a zinc alloy electrode inhibited corrosion of the zinc when the zinc alloy contained at least one metal selected from bismuth, lithium, calcium and aluminum and was free of mercury, lead, cadmium, indium and thallium. In Japanese patent application Kokai H9-055207, Ninakata et al. report that an alkaline battery having a zinc alloy powder negative electrode provides controlled gas generation without the use of mercury or lead when the zinc alloy contains certain amounts of indium, aluminum, bismuth, magnesium; an additional indium compoundxe2x80x94such as indium oxide; and at least one element selected from lithium, potassium and calcium.
Another zinc-based electrode material that is free of mercury, cadmium and lead was reported by Yasumura et al. in Japanese patent application Kokai H9-259877, where it was disclosed that the zinc-based alloy must contain certain amounts of oxygen, indium, bismuth and aluminum; certain amounts of oxygen, indium, bismuth and magnesium; or certain amounts of oxygen, indium and nickel. It was reported that batteries made with the claimed zinc-based electrode alloys had improved self discharge and load discharge capacities.
Murajima et al., in Japanese patent application Kokai H4-368776 reported a mercury-free zinc alloy powder that was capable of controlling hydrogen generation and of improving battery properties at low temperatures. The zinc alloy included a certain amount of at least one metal selected from aluminum, gallium, indium, thallium, magnesium, calcium, strontium, tin, lead, bismuth, cadmium, silver, tellurium, copper, nickel, barium and germanium.
In Japanese patent application Kokai H5-182661, Murashima et al. disclose a zinc alloy powder with no added mercury or lead that consisted of zinc with certain amounts of at least one type of metal selected from lead, indium, bismuth, aluminum and calcium.
Despite the promising work that has recently been reported in this field, however, a need still remains for an alkaline cell electrode material that is substantially free of both lead and mercury and that provides gassing properties that are improved over pure zinc metal. It would be particularly useful if the gassing properties of the electrode material were at least comparable with the gassing properties of presently commercial lead-containing zinc electrode powder.
Although the gas generation properties of various electrode materials has been the major focus of much of the work directed at the improvement of these materials, the discharge capacity of batteries that use the materials is also important to the ultimate performance of the battery. Thus, it would be even more useful if an electrode material that is substantially free of both lead and mercury and that had improved gassing properties would also provide a battery having a discharge capacity that was at least comparable to a battery that uses a presently commercial lead-containing zinc electrode powder.
Briefly, therefore, the present invention is directed to a novel zinc alloy for use in batteries and having improved gassing properties, the alloy comprising a bismuth-indium (BiIn) intermetallic compound in an amount sufficient to improve the gassing properties of the alloy; and with the major part of the alloy being zinc that is substantially free of lead and mercury.
The present invention is also directed to a novel method for preparing a zinc alloy for use in alkaline batteries having improved gassing properties, the method comprising mixing together into a molten mixture, bismuth and indium in a weight ratio of from about 40/60 to about 70/30; and with the major part being zinc that is substantially free of lead and mercury; and solidifying the molten mixture under conditions so that at least some of the bismuth and indium form a bismuth-indium (BiIn) intermetallic compound in an amount sufficient to improve the gassing properties of the alloy.
The present invention is also directed to a novel battery of the type having an anode of zinc alloy, wherein the zinc alloy comprises bismuth and indium in a BiIn intermetallic compound; and with the major part being zinc that is substantially free of lead and mercury.
The present invention is also directed to a novel method for making a battery having an anode that is substantially free of lead and mercury and has low gassing, said method comprising preparing an anode from zinc alloy that comprises bismuth and indium in a BiIn intermetallic compound and with the major part being zinc that is substantially free of lead and mercury; and using the anode to form a battery.
Among the several advantages found to be achieved by the present invention, therefore, may be noted the provision of an alkaline cell electrode material that is substantially free of both lead and mercury; the provision of such an electrode material that provides gassing properties that are improved over pure zinc metal; the provision of such an electrode material having gassing properties that are at least comparable with the gassing properties of presently commercial lead-containing zinc electrode powder; and the provision of such an alkaline cell electrode material that provides a battery having a discharge capacity that is at least comparable to a battery that uses a presently commercial lead-containing zinc electrode powder.